In the past, the gain of each frequency band of a music signal is generally manipulated using an equalizer. At this occasion, when the gain of a particular frequency band is amplified, the amplitude of a signal that is output from the equalizer, i.e., the amplitude of a music signal of which gain has been manipulated, is beyond a recording range, and as a result, in terms of audibility, uncomfortable clip distortion may occur.
Pre-gain control processing and auto gain control processing are known as techniques for reducing such clip distortion.
In the pre-gain control processing, the amount of gain that is clipped is estimated in advance, and after a music signal is attenuated on the basis of the amount of gain, the attenuated music signal is input into an equalizer. At this occasion, the amount of attenuation of the music signal is determined by allowing a user to manually operate an attenuation knob.
In the auto gain control processing, the gain control is performed adaptively on the music signal right before input into the equalizer or right after output from the equalizer in order to prevent the music signal from being clipped in the gain operation of the music signal with the equalizer. More specifically, in a section where the clip distortion occurs, the signal waveform of the music signal is forcibly deformed so that the amplitude of the signal is not beyond the recording range.
For example, as shown in the upper side of FIG. 1, suppose that the music signal having the waveform indicated by arrow Q11 is input into the equalizer 11, and the gain adjustment of particular frequency band is done, and a music signal having a waveform indicated by arrow Q12 is obtained as output. In the waveform indicated by arrow Q12, a portion of the waveform where the amplitude is excessively amplified, i.e., a portion indicated by a dotted line, is clipped and has distorted waveform. When such clip distortion occurs, the sound is destroyed when the music signal is played, and the quality of the sound is degraded.
Accordingly, an apparatus that performs the pre-gain control processing and the auto gain control processing is provided with an attenuation circuit 12 in a stage prior to the equalizer 11 as shown in the lower side in the figure, and the music signal of which waveform is to be processed indicated by arrow Q21 is input into the attenuation circuit 12. When the attenuation processing is performed on the music signal by the attenuation circuit 12, a music signal having a waveform of which gain has been attenuated indicated by arrow Q22 is obtained, and this music signal is provided to the equalizer 11 so that the gain is adjusted.
Then, with the gain adjustment of the music signal that is performed by the equalizer 11, a music signal having a waveform indicated by arrow Q23 is obtained as an output. The gain of the music signal thus obtained is attenuated by the attenuation circuit 12 in advance in accordance with the gain that is excessively amplified by the equalizer 11, and therefore, the clip distortion can be alleviated.
In addition, in a technique for reducing clip distortion, peaking filters of multiple frequency bands are connected in series (for example, see Patent Document 1). In this technique, in later stages, filter processing is performed on the signal with the peaking filters for higher frequency, and therefore, harmonics components of distortion generated by clipping of the signal is less likely to be amplified by filters in later stages.